Physiological responses to fluctuating thermal and hydration regimes in the chill susceptible insect, Thaumatotibia leucotreta

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Date
2013Author
Boardman, L.
Sorensen, J.G.
Terblanche, J.S.
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Fluctuating thermal regimes (FTR), consisting of cycles between stressful low and benign temperatures,
are known to improve survival and fecundity in a variety of insects. By contrast, fluctuating hydration
regimes (FHR) consisting of cycles between dehydrating and benign conditions have been less comprehensively
researched. Hypothetically, either repeated stress accumulates damage and reduces survival,
or the recovery periods may act as a protective mechanism by allowing low temperature- or dehydration-
induced damage to be repaired. Using false codling moth (Thaumatotibia leucotreta) larvae, we investigated
whether FTR and FHR resulted in protection, or accumulated damage, at the cellular and wholeorganism
levels. Time- and age-matched controls were used to verify that the effects were due to the
fluctuating stressors and not age- or time-dependent responses. Results showed that larval body
water-(BWC) and lipid content (BLC) remained unchanged in response to FTR. Importantly though, FTR
are protective when compared to constant low temperature exposures, potentially due to an increase
in heat shock protein 70 (HSP70). However, larvae may suffer long-term fitness consequences compared
to constant benign exposures. Results for FHR appear equivocal when compared to constant controls, due
to high survival rates for all experiments, although the physiological responses to FHR included a
decrease in larval BWC and BLC, a decrease in cuticular water loss rates, and a depletion of HSP70 during
the final dehydration cycle. In conclusion, it appears that fluctuating stressors are protective in T. leucotreta
when compared to constant stress conditions, likely through regulation of whole-animal metabolic
rate and HSP70, although other mechanisms (e.g. ion homeostasis) are also implicated.